Electoral Registers of England and Wales, 1832-1932.
This whopper collection holds more than 5.4million images and around 220 million names, so it's an important record set for anyone whose Irish ancestors migrated to England or Wales.
Registers were compiled annually, so it's possible to track families way past the 1911 census, and possibly even fill in gaps between earlier censuses.
Bear in mind that the right to vote was severely restricted during much of the period covered by the collection. It was only after World War One that property ownership ceased to be the main criteria for a vote
Considerably smaller collections of electoral registers have also been released for Scotland and Ireland.
The Electoral Registers of Ireland, 1885 and 1886: This record set consists of just over 3,200 entries from the counties of Armagh, Fermanagh, Down, Limerick, Mayo, Meath, Tyrone, Roscommon, Westmeath, Wexford and Wicklow. (Of these, only Wicklow features in my family tree and I was pleased to find my great great grandfather's entry in High Street, Wicklow Town. This is where he was living at the time of the 1901 census, so this is not an earth-shattering revelation, but it suggests this was the permanent family home – where my gt grandmother was brought up – rather than a short-term tenancy.)
The Electoral Registers of Scotland, 1864-1931: The name of this record set is a little misleading as the entries cover only the county of Linlithgowshire (present day West Lothian). However, it has a good spread of years and holds more than 20,000 entries.